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1
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2 A working guide to kestrel
3 ==========================
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4
5 Kestrel is a very simple message queue that runs on the JVM and uses the
6 memcache protocol (with some extensions) to talk to clients.
7
8 A single kestrel server has a set of queues identified by a name, which is
9 also the filename of that queue's journal file (usually in
10 `/var/spool/kestrel`). Each queue is a strictly-ordered FIFO of "items" of
11 binary data. Usually this data is in some serialized format like JSON or
12 ruby's marshal format.
13
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14 Generally queue names should be limited to alphanumerics `[A-Za-z0-9]`, dash
15 (`-`) and underline (`_`). In practice, kestrel doesn't enforce any
16 restrictions other than the name can't contain slash (`/`) because that can't
17 be used in filenames, squiggle (`~`) because it's used for temporary files,
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18 plus (`+`) because it's used for fanout queues, and dot (`.`) because it's
19 reserved for future use. Queue names are case-sensitive, but if you're running
20 kestrel on OS X or Windows, you will want to refrain from taking advantage of
21 this, since the journal filenames on those two platforms are *not*
22 case-sensitive.
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23
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24 A cluster of kestrel servers is like a memcache cluster: the servers don't
25 know about each other, and don't do any cross-communication, so you can add as
26 many as you like. Clients have a list of all servers in the cluster, and pick
27 one at random for each operation. In this way, each queue appears to be spread
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28 out across every server, with items in a loose ordering.
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29
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30 When kestrel starts up, it scans the journal folder and creates queues based
31 on any journal files it finds there, to restore state to the way it was when
32 it last shutdown (or was killed or died). New queues are created by referring
33 to them (for example, adding or trying to remove an item). A queue can be
34 deleted with the "delete" command.
35
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36
37 Configuration
38 -------------
39
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40 The config files for kestrel are scala expressions loaded at runtime, usually
41 from `production.scala`, although you can use `development.scala` by passing
42 `-Dstage=development` to the java command line.
43
44 The config file evaluates to a `KestrelConfig` object that's used to configure
45 the server as a whole, a default queue, and any overrides for specific named
46 queues. The fields on `KestrelConfig` are documented here with their default
47 values:
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48 [KestrelConfig.html](http://robey.github.com/kestrel/doc/main/api/net/lag/kestrel/config/KestrelConfig.html)
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49
50 To confirm the current configuration of each queue, send "dump_config" to
51 a server (which can be done over telnet).
52
53 To reload the config file on a running server, send "reload" the same way.
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54 You should immediately see the changes in "dump_config", to confirm. Reloading
55 will only affect queue configuration, not global server configuration. To
56 change the server configuration, restart the server.
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57
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58 Logging is configured according to `util-logging`. The logging configuration
59 syntax is described here:
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60 [util-logging](https://github.com/twitter/util/blob/master/util-logging/README.markdown)
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61
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62 Per-queue configuration is documented here:
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63 [QueueBuilder.html](http://robey.github.com/kestrel/doc/main/api/net/lag/kestrel/config/QueueBuilder.html)
64
65
66 Full queues
67 -----------
68
69 A queue can have the following limits set on it:
70
71 - `maxItems` - total items in the queue
72 - `maxSize` - total bytes of data in the items in the queue
73
74 If either of these limits is reached, no new items can be added to the queue.
75 (Clients will receive an error when trying to add.) If you set
76 `discardOldWhenFull` to true, then all adds will succeed, and the oldest
77 item(s) will be silently discarded until the queue is back within the item
78 and size limits.
79
80 `maxItemSize` limits the size of any individual item. If an add is attempted
81 with an item larger than this limit, it always fails.
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82
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83
84 The journal file
85 ----------------
86
87 The journal file is the only on-disk storage of a queue's contents, and it's
88 just a sequential record of each add or remove operation that's happened on
89 that queue. When kestrel starts up, it replays each queue's journal to build
90 up the in-memory queue that it uses for client queries.
91
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92 The journal file is rotated in one of two conditions:
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93
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94 1. the queue is empty and the journal is larger than `defaultJournalSize`
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95
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96 2. the journal is larger than `maxJournalSize`
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97
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98 For example, if `defaultJournalSize` is 16MB (the default), then if the queue
99 is empty and the journal is larger than 16MB, it will be truncated into a new
100 (empty) file. If the journal is larger than `maxJournalSize` (1GB by default),
101 the journal will be rewritten periodically to contain just the live items.
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102
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103 You can turn the journal off for a queue (`keepJournal` = false) and the queue
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104 will exist only in memory. If the server restarts, all enqueued items are
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105 lost. You can also force a queue's journal to be sync'd to disk periodically,
106 or even after every write operation, at a performance cost, using
107 `syncJournal`.
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108
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109 If a queue grows past `maxMemorySize` bytes (128MB by default), only the
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110 first 128MB is kept in memory. The journal is used to track later items, and
111 as items are removed, the journal is played forward to keep 128MB in memory.
112 This is usually known as "read-behind" mode, but Twitter engineers sometimes
113 refer to it as the "square snake" because of the diagram used to brainstorm
114 the implementation. When a queue is in read-behind mode, removing an item will
115 often cause 2 disk operations instead of one: one to record the remove, and
116 one to read an item in from disk to keep 128MB in memory. This is the
117 trade-off to avoid filling memory and crashing the JVM.
118
119
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120 Item expiration
121 ---------------
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122
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123 When they come from a client, expiration times are handled in the same way as
124 memcache: if the number is small (less than one million), it's interpreted as
125 a relative number of seconds from now. Otherwise it's interpreted as an
126 absolute unix epoch time, in seconds since the beginning of 1 January 1970
127 GMT.
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128
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129 Expiration times are immediately translated into an absolute time, in
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130 *milliseconds*, and if it's further in the future than the queue's `maxAge`,
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131 the `maxAge` is used instead. An expiration of 0, which is usually the
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132 default, means an item never expires.
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133
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134 Expired items are flushed from a queue whenever a new item is added or
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135 removed. Additionally, if the global config option `expirationTimerFrequency`
136 is set, a background thread will periodically remove expired items from the
137 head of each queue. The provided `production.conf` sets this to one second.
138 If this is turned off, an idle queue won't have any items expired, but you
139 can still trigger a check by doing a "peek" on it.
140
141 Normally, expired items are discarded. If `expireToQueue` is set, then
142 expired items are moved to the specified queue just as if a client had put
143 it there. The item is added with no expiration time, but that can be
144 overridden if the new queue has a default expiration policy.
145
146 To prevent stalling the server when it encounters a swarm of items that all
147 expired at the same time, `maxExpireSweep` limits the number of items that
148 will be removed by the background thread in a single round. This is primarily
149 useful as a throttling mechanism when using a queue as a way to delay work.
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150
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151
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152 Fanout Queues
153 -------------
154
155 If a queue name has a `+` in it (like "`orders+audit`"), it's treated as a
156 fanout queue, using the format `<parent>+<child>`. These queues belong to a
157 parent queue -- in this example, the "orders" queue. Every item written into
158 a parent queue will also be written into each of its children.
159
160 Fanout queues each have their own journal file (if the parent queue has a
161 journal file) and otherwise behave exactly like any other queue. You can get
162 and peek and even add items directly to a child queue if you want. It uses the
163 parent queue's configuration instead of having independent child queue
164 configuration blocks.
165
166 When a fanout queue is first referenced by a client, the journal file (if any)
167 is created, and it will start receiving new items written to the parent queue.
168 Existing items are not copied over. A fanout queue can be deleted to stop it
169 from receiving new items.
170
171
172 Memcache commands
173 -----------------
174
175 - `SET <queue-name> <flags (ignored)> <expiration> <# bytes>`
176
177 Add an item to a queue. It may fail if the queue has a size or item limit
178 and it's full.
179
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180 - `GET <queue-name>[options]`
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181
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182 Remove an item from a queue. It will return an empty response immediately if
183 the queue is empty. The queue name may be followed by options separated
184 by `/`:
185
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186 - `/t=<milliseconds>`
187
188 Wait up to a given time limit for a new item to arrive. If an item arrives
189 on the queue within this timeout, it's returned as normal. Otherwise,
190 after that timeout, an empty response is returned.
191
192 - `/open`
193
194 Tentatively remove an item from the queue. The item is returned as usual
195 but is also set aside in case the client disappears before sending a
196 "close" request. (See "Reliable Reads" below.)
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197
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198 - `/close`
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199
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200 Close any existing open read. (See "Reliable Reads" below.)
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201
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202 - `/abort`
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203
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204 Cancel any existing open read, returing that item to the head of the
205 queue. It will be the next item fetched. (See "Reliable Reads" below.)
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206
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207 - `/peek`
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208
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209 Return the first available item from the queue, if there is one, but don't
210 remove it. You can't combine this with any of the reliable read options.
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211
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212 For example, to open a new read, waiting up to 500msec for an item:
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213
214 GET work/t=500/open
215
216 Or to close an existing read and open a new one:
217
218 GET work/close/open
219
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220 - `DELETE <queue-name>`
221
222 Drop a queue, discarding any items in it, and deleting any associated
223 journal files.
224
225 - `FLUSH <queue-name>`
226
227 Discard all items remaining in this queue. The queue remains live and new
228 items can be added. The time it takes to flush will be linear to the current
229 queue size, and any other activity on this queue will block while it's being
230 flushed.
231
232 - `FLUSH_ALL`
233
234 Discard all items remaining in all queues. The queues are flushed one at a
235 time, as if kestrel received a `FLUSH` command for each queue.
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236
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237 - `VERSION`
238
239 Display the kestrel version in a way compatible with memcache.
240
241 - `SHUTDOWN`
242
243 Cleanly shutdown the server and exit.
244
245 - `RELOAD`
246
247 Reload the config file and reconfigure all queues. This should have no
248 noticable effect on the server's responsiveness.
249
250 - `DUMP_CONFIG`
251
252 Dump a list of each queue currently known to the server, and list the config
253 values for each queue. The format is:
254
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255 queue 'master' {
256 max_items=2147483647
257 max_size=9223372036854775807
258 max_age=0
259 max_journal_size=16277216
260 max_memory_size=134217728
261 max_journal_overflow=10
262 max_journal_size_absolute=9223372036854775807
263 discard_old_when_full=false
264 journal=true
265 sync_journal=false
266 }
267
268 The last queue will be followed by `END` on a line by itself.
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269
270 - `STATS`
271
272 Display server stats in memcache style. They're described below.
273
274 - `DUMP_STATS`
275
276 Display server stats in a more readable style, grouped by queue. They're
277 described below.
278
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279 - `MONITOR <queue-name> <seconds>`
280
281 Monitor a queue for a time, fetching any new items that arrive. Clients
282 are queued in a fair fashion, per-item, so many clients may monitor a
283 queue at once. After the given timeout, a separate `END` response will
284 signal the end of the monitor period. Any fetched items are open
285 transactions (see "Reliable Reads" below), and should be closed with
286 `CONFIRM`.
287
288 - `CONFIRM <queue-name> <count>`
289
290 Confirm receipt of `count` items from a queue. Usually this is the response
291 to a `MONITOR` command, to confirm the items that arrived during the monitor
292 period.
293
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294
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295 Reliable reads
296 --------------
297
298 Normally when a client removes an item from the queue, kestrel immediately
299 discards the item and assumes the client has taken ownership. This isn't
300 always safe, because a client could crash or lose the network connection
301 before it gets the item. So kestrel also supports a "reliable read" that
302 happens in two stages, using the `/open` and `/close` options to `GET`.
303
304 When `/open` is used, and an item is available, kestrel will remove it from
305 the queue and send it to the client as usual. But it will also set the item
306 aside. If a client disconnects while it has an open read, the item is put back
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307 into the queue, at the head, so it will be the next item fetched. Only one
308 item can be "open" per client connection.
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309
310 A previous open request is closed with `/close`. The server will reject any
311 attempt to open another read when one is already open, but it will ignore
312 `/close` if there's no open request, so that you can add `/close` to every
313 `GET` request for convenience.
314
315 If for some reason you want to abort a read without disconnecting, you can use
316 `/abort`. But because aborted items are placed back at the head of the queue,
317 this isn't a good way to deal with client errors. Since the error-causing item
318 will always be the next one available, you'll end up bouncing the same item
319 around between clients instead of making progress.
320
321 There's always a trade-off: either potentially lose items or potentially
322 receive the same item multiple times. Reliable reads choose the latter option.
323 To use this tactic successfully, work items should be idempotent, meaning the
324 work could be done 2 or 3 times and have the same effect as if it had been
325 done only once (except wasting some resources).
326
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327 Example:
328
329 GET dirty_jobs/close/open
330 (receives job 1)
331 GET dirty_jobs/close/open
332 (closes job 1, receives job 2)
333 ...etc...
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334
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335
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336 Server stats
337 ------------
338
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339 Global stats reported by kestrel are:
340
341 - `uptime` - seconds the server has been online
342 - `time` - current time in unix epoch
343 - `version` - version string, like "1.2"
344 - `curr_items` - total of items waiting in all queues
345 - `total_itmes` - total of items that have ever been added in this server's
346 lifetime
347 - `bytes` - total byte size of items waiting in all queues
348 - `curr_connections` - current open connections from clients
349 - `total_connections` - total connections that have been opened in this
350 server's lifetime
351 - `cmd_get` - total `GET` requests
352 - `cmd_set` - total `SET` requests
353 - `cmd_peek` - total `GET/peek` requests
354 - `get_hits` - total `GET` requests that received an item
355 - `get_misses` - total `GET` requests on an empty queue
356 - `bytes_read` - total bytes read from clients
357 - `bytes_written` - total bytes written to clients
358
359 For each queue, the following stats are also reported:
360
361 - `items` - items waiting in this queue
362 - `bytes` - total byte size of items waiting in this queue
363 - `total_items` - total items that have been added to this queue in this
364 server's lifetime
365 - `logsize` - byte size of the queue's journal file
366 - `expired_items` - total items that have been expired from this queue in this
367 server's lifetime
368 - `mem_items` - items in this queue that are currently in memory
369 - `mem_bytes` - total byte size of items in this queue that are currently in
370 memory (will always be less than or equal to `max_memory_size` config for
371 the queue)
372 - `age` - time, in milliseconds, that the last item to be fetched from this
373 queue had been waiting; that is, the time between `SET` and `GET`; if the
374 queue is empty, this will always be zero
375 - `discarded` - number of items discarded because the queue was too full
376 - `waiters` - number of clients waiting for an item from this queue (using
377 `GET/t`)
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378 - `open_transactions` - items read with `/open` but not yet confirmed
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379
380
381 Kestrel as a library
382 --------------------
383
384 You can use kestrel as a library by just sticking the jar on your classpath.
385 It's a cheap way to get a durable work queue for inter-process or inter-thread
386 communication. Each queue is represented by a `PersistentQueue` object:
387
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388 class PersistentQueue(val name: String, persistencePath: String,
389 @volatile var config: QueueConfig, timer: Timer,
390 queueLookup: Option[(String => Option[PersistentQueue])]) {
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391
392 and must be initialized before using:
393
394 def setup(): Unit
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395
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396 specifying the path for the journal files (if the queue will be journaled),
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397 the name of the queue, a `QueueConfig` object (derived from `QueueBuilder`),
398 a timer for handling timeout reads, and optionally a way to find other named
399 queues (for `expireToQueue` support).
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400
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401 To add an item to a queue:
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402
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403 def add(value: Array[Byte], expiry: Option[Time]): Boolean
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404
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405 It will return `false` if the item was rejected because the queue was full.
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406
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407 Queue items are represented by a case class:
408
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409 case class QItem(addTime: Time, expiry: Option[Time], data: Array[Byte], var xid: Int)
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410
411 and several operations exist to remove or peek at the head item:
412
413 def peek(): Option[QItem]
414 def remove(): Option[QItem]
415
416 To open a reliable read, set `transaction` true, and later confirm or unremove
417 the item by its `xid`:
418
419 def remove(transaction: Boolean): Option[QItem]
420 def unremove(xid: Int)
421 def confirmRemove(xid: Int)
422
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423 You can also asynchronously remove or peek at items using futures.
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424
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425 def waitRemove(deadline: Option[Time], transaction: Boolean): Future[Option[QItem]]
426 def waitPeek(deadline: Option[Time]): Future[Option[QItem]]
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427
428 When done, you should close the queue:
429
430 def close(): Unit
431 def isClosed: Boolean
432
433 Here's a short example:
434
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435 var queue = new PersistentQueue("work", "/var/spool/kestrel", config, timer, None)
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436 queue.setup()
437
438 // add an item with no expiration:
439 queue.add("hello".getBytes, 0)
440
441 // start to remove it, then back out:
442 val item = queue.remove(true)
443 queue.unremove(item.xid)
444
445 // remove an item with a 500msec timeout, and confirm it:
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446 queue.waitRemove(500.milliseconds.fromNow, true)() match {
447 case None =>
448 println("nothing. :(")
449 case Some(item) =>
450 println("got: " + new String(item.data))
451 queue.confirmRemove(item.xid)
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452 }
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453
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454 queue.close()
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